flock, fcntl record locks

There were none: fcntl answered F_SETLK with EINVAL and flock was not a
system call. Alpine's apk, which runs unmodified on SchOS through
Alpine's own musl, locks its database before reading it and stopped
there.

kernel/fs/lock.c keeps one table of advisory locks keyed by (dev, inum):
flock() for the whole file, owned by the open file description;
F_SETLK/F_SETLKW/F_GETLK for byte ranges, owned by the process and
released when it closes any descriptor on the file or exits; and the
F_OFD_* forms, ranges owned by the description. flock and record locks
do not see each other, as on Linux. Waiters sleep until any release and
are interrupted by a signal.

cmd/locktest checks each between a parent and a child, including range
splitting, F_GETLK's report, and a blocked waiter being woken.

shell.nix gains pkg-config, meson, ninja, m4, bison, flex and xz, for
the X ports.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

include/uapi/fcntl.h

@@ -39,5 +39,12 @@
 #define F_GETFL          3
 #define F_SETFL          4
 #define F_DUPFD_CLOEXEC  1030  // Linux value
+// Record locks; see kernel/fs/lock.c.
+#define F_GETLK          5
+#define F_SETLK          6
+#define F_SETLKW         7
+#define F_OFD_GETLK      36
+#define F_OFD_SETLK      37
+#define F_OFD_SETLKW     38
 
 #define FD_CLOEXEC 1